1. Field of the Invention
The present invention relates to dam communications networks, and more particularly to a system for measuring transit delay in a packet network.
2. Description of the Related Art
In many data communications networks, data is generated in high intensity bursts that are often separated by idle periods therebetween. The average data rate may, therefore, be much lower than the peak rate. An economic way of transmitting this type of intermittent (bursty) data is to assemble the data into what is known in the art as "packets" and interspersing packets from several different channels on a single, physical communication path. This communication path is therefore occupied by a particular packet only for the time corresponding to the packet length, and the communication path is then available for use by other packets that may be between different data terminals.
A packet refers to a group of bits that are switched in the network as an integral unit. A packet may also include a header that is used to identify that packet. A packet is formatted in a particular way and may contain data, and destination, origination and control information.
In a typical packet-switching network, a number of users can be interconnected. Packet switches are utilized in the network to sort packets received from one circuit and direct these packets to another circuit according to the header information for each packet. A typical packet switch is AT&T's 1 PSS.TM. packet switch which implements the CCITT X.25 packet-switching protocol.
In any data communications network, a primary concern in the performance of the network is the time in which information is processed and transmitted back to the user. This time is typically referred to as response time or network transit delay. Network transit delay in a packet-switching network may be a function of data transmission speed, propagation time, distance through the network, packet-switch processing time and queuing delay.
To evaluate the performance of a packet network, transit delay is generally measured from the time the last bit of a particular packet enters the provider's originating central office equipment (e.g., originating packet switch) to the time the first bit of the packet leaves the provider's destination central office equipment (e.g., destination packet switch).
In designing and maintaining data communications systems, it is essential to minimize network transit delays. In fact, most customers today have strict requirements regarding the permissible transit delay in the network. Customers will often require the provider of the data communications network to pay a penalty if the transit delay exceeds a predetermined level.
Therefore, it is important for the design and maintenance of a data communications system to effectively measure transit delay in the packet network. Moreover, customers of the system frequently require periodic measurements of transit delay to ascertain whether the system is performing within the limits of the customers' service contracts.
There are currently a number of measurement systems used today to estimate or measure network transit delay. These measurement systems generally fall into two categories and are illustrated in FIG. 1.
The first type of measurement system is a protocol analyzer 20 which includes hardware connected to the packet network 10 for analyzing and measuring transit delay. Transit delay is measured by connecting a protocol analyzer 20 to a desired measurement point in the network 10 and essentially time stamping a particular packet to determine the actual transit delay of that particular packet.
This type of measurement system, however, can be extremely expensive because it requires a protocol analyzer 20 at each desired measurement point. Thus, if a customer has numerous access points to the network 10, as they often do, protocol analyzers 20 would be required at each point of access into the network 10.
In addition to the expense associated with requiring a protocol analyzer 20 at each desired measurement point, each protocol analyzer 20 must be located in the service node (i.e., the packet switch) in the network 10, and also requires space for locating the protocol analyzer 20, power for operating the equipment, and the ability to access the equipment for installation and maintenance purposes.
The second type of system for measuring transit delay is through the use of measurement software 22 that may exist in the customer's end equipment and/or at various locations in the packet network 10. The measurement software 22, however, must be developed, tested and integrated into the network 10 and frequently requires updating (reintegration) each time the software in the packet network 10 is updated, changed or otherwise modified (software maintenance).
It is, therefore, desirable to provide a cost effective, non-invasive measurement system to estimate transit delay in a packet network 10 that does not require hardware or measuring software to be added to the network 10.